1. Field of the Invention
The present invention relates to a diffractive optical element that reduces the generation of unnecessary light.
2. Description of the Related Art
In a diffractive optical element used as a lens of an optical system, it is known that two diffractive grating are adhesively disposed and a material constituting each diffractive grating and a grating height are appropriately set to obtain a high diffraction efficiency in a wide wavelength range. Light entering a grating wall surface of the diffractive optical element which is provided with the grating surface and the grating wall surface acts differently from the grating surface, for example it is reflected or refracted on the grating wall surface, and the unnecessary light (flare) is generated. Japanese Patent Laid-Open No. 2003-240931 and Japanese Patent Laid-Open No. 2004-126394 disclose a diffractive optical element which is configured so as to reduce the unnecessary light on the grating wall surface. The diffractive optical element disclosed in Japanese Patent Laid-Open No. 2003-240931 and Japanese Patent Laid-Open No. 2004-126394 includes a light shielding member such as an opaque film or an absorption film on the grating wall surface, and it shields the light entering the grating wall surface and reduces the unnecessary light generated on the grating wall surface. Japanese Patent Laid-Open No. 2009-217139 discloses a calculation of the diffraction efficiency using the rigorous coupled wave analysis (RCWA).
However, in Japanese Patent Laid-Open No. 2003-240931 and Japanese Patent Laid-Open No. 2004-126394 do not sufficiently disclose the generation cause of the unnecessary light and do not disclose a specific configuration of the extent of the transmission, the reflection, and the absorption, with regard to the absorption film. Therefore, the unnecessary light cannot be effectively reduced.